U.S. Pat. No. 8,353,871 describes a drug delivery device that includes a roller with hollow needles and a drug reservoir for delivering the drug through the needles as the roller is passed over the skin. The roller is rotatably mounted on an axle. As the roller rotates about the axle, contents of the reservoir pass through the hollow needles. The roller and a piston slide with respect to other as the roller rotates about the axle. The relative sliding motion causes the piston to push contents of the reservoir through the needles.